Hummer, a sergeant at Roxbury, is accused of failing to stop other correctional officers from beating Davis, failing to summon medical attention, conspiring to cover up the incident, and misleading state investigators and administrative judges.

Nine officers were originally charged in state court but only two were convicted. Federal authorities picked up the case, and 12 have since pleaded guilty. Two others await trial.

Federal prosecutor Christine Mary Siscaretti said in court that Hummer lied when he told investigators that he saw "nothing" in the cell.

But Clarke F. Ahlers, Hummer's attorney, said the prosecutor is misconstruing Hummer's statements. He saw guards in Davis' cell. He just didn't witness them punching and kicking him.

In March 2008, Davis suffered nose, rib, vertebrae and facial fractures, as well as bruised eyes. Siscaretti said eight prison guards beat Davis in retaliation for his punching a guard in the nose the day before.

None of those facts are in contention by either side. And defense and prosecution agree that guards were in or around Davis' cell when Hummer walked by.

Prosecutors said he went to the Administrative Segregation Area — or "Asia," as guards called it — where Davis was being held to watch the beating. His attorney said Hummer was simply looking for some lost gloves.

Siscaretti told jurors in opening statements that the guards involved in the beating will testify to Hummer's presence and that "he chose to do nothing." She further alleged that while he acknowledged Davis had "had enough" during the beating he also felt justified watching, telling guards "I had to get mine."

Ahlers, however, attempted to sow doubt in jurors' minds by saying Hummer never denied seeing the guards, but told investigators that he "saw nothing unusual."

"What Josh Hummer is expressing is that he did not see a standard-of-conduct violation," he said.

He said Hummer saw Davis underneath his bunk with his head turned toward the wall and guard Tyson Hinckle squatting down beside him — a scene that did not seem unusual in the prison, which houses about 2,000 prisoners.

Ahlers went on to say that prosecution witnesses were "bought" because they accepted plea deals in return for their cooperation against Hummer. He also said several of them have either lied during the investigation or have given conflicting accounts.

"What the jury has to decide — and the crux of this case — is what Josh Hummer saw," Ahlers said.

U.S. District Judge James K. Bredar said the trial is expected to last two weeks.

Just getting accepted to Johns Hopkins University was an accomplishment for Jeremy Huber. Getting recruited to play lacrosse for a program that has won nine national championships was another thing entirely.